The present invention relates in general to a relative steering angle sensor for detecting rotation of a steering wheel in a vehicle wherein a relative position offset is determined according to a dynamically determined steering wheel center position, and, more specifically, to sensor diagnostics for detecting short circuits or open circuits and for modifying the process for maintaining the relative position offset during fault conditions.
Steering angle sensors are employed on vehicles for a variety of purposes, such as yaw stability control (YSC) systems which sense vehicle operation and automatically apply braking forces to improve vehicle stability and control (e.g., reduce understeer or oversteer). To address certain drawbacks of steering angle sensors that incorporated an absolute position angle reference (e.g., inaccuracy due to steering mechanism wear, influence of uneven road surface, etc.), techniques were developed for using steering angle sensors that sense rotation without having a fixed reference. A xe2x80x9csoftxe2x80x9d position reference is determined by dynamically adjusting a relative position offset for each driving cycle beginning at xe2x80x9ckey onxe2x80x9d of the vehicle in response to predetermined vehicle parameters from other sensors or actuators in the vehicle. These parameters include vehicle speed and yaw rate, for example.
Angle sensors employed for monitoring steering angle changes are typically comprised of quadrature pulse generators wherein the relative phase between two pulse trains identifies the direction of steering wheel rotation. The pulses are typically square waves produced by optical sensors wherein light beams are interrupted by a slotted disc that rotates with the steering wheel. Wiring from the sensor to a control module includes a connector which may typically be integral with a sensor housing.
The pulse signals from the sensor transition between a high signal state and a low signal state. Because of voltage drops across devices in the sensor circuitry, the high signal state has a voltage slightly less than the supply voltage Vss and the low signal state has a voltage slightly higher than ground. The wiring to a sensor typically includes separate wires for Vss, ground, and each of the quadrature signals (e.g., phase A and phase B).
Electronic control modules typically include diagnostic routines for detecting faulted sensors. Potential failures associated with an angle sensor include intermittent connections caused by a loose connector, shorts to ground, and shorts to battery. These faults can be detected by monitoring voltage on the quadrature signal lines. Each voltage is compared to acceptable voltage ranges for the high signal state and the low signal state (i.e., voltages other than Vss and ground), and a fault is detected whenever the voltages fall outside these ranges.
In order to provide noise immunity, a fault is not detected until an unacceptable voltage has persisted for a predetermined length of time (e.g., 100 milliseconds). However, intermittent faults can appear and then disappear on a shorter time scale. It has been discovered that such intermittent faults can go undetected and that they can cause the loss of rotation pulses from the sensor resulting in accumulated error in the determined steering angle. Incorrect steering angle can lead to false activation of the YSC system.
The present invention has the advantage of detecting intermittent sensor signal faults very quickly without complete shutdown of the sensor unless a fault persists for a sufficient amount of time.
In one aspect, the present invention provides a method of determining a relative position offset for a steering angle sensor in a vehicle. The relative position offset is iteratively adjusted in response to predetermined vehicle parameters. A voltage from the angle sensor is compared to a fault threshold. A duration of time is measured during which the comparing step indicates a fault. If a fault is indicated for a duration of time greater that a first predetermined duration, then the adjusting step is suspended. If the fault ceases to be indicated prior to reaching a second predetermined duration, then the adjusting step is resumed. If the fault continues to be indicated until reaching the second predetermined duration, then a faulted sensor signal is generated.